This invention relates to a jet pump for a marine propulsion system and more particularly to an improved twin impeller, jet propulsion system.
The use of jet propulsion systems, frequently referred to also as "jet pumps," for powering watercraft is well known and well accepted. These units have a number of advantages over more conventional propeller driven watercraft.
The conventional type of jet propulsion unit most commonly employed includes an outer housing having a water inlet opening through which water is drawn. An impeller is disposed in the water inlet and is driven by an external prime mover for drawing water through the water inlet and discharging it through a discharge nozzle for providing a propulsion force. Normally these units employ a series of fixed straightening vanes behind the impeller for reducing the swirling action of the water discharged so as to improve the performance.
Although these units are quite effective, they do have some disadvantages. For example, when accelerating, there is a likelihood of cavitation occurring and this can significantly reduce the efficiency of the unit. Also, the straightening vanes provide a resistance to water flow and reduce the propulsion effectiveness of the unit.
It has been proposed, therefore, to employ a pair of impellers that rotate in opposite directions for pumping the water. However, the systems that have been previously proposed for this purpose have been quite complicated and not totally effective.
It is, therefore, a principal object of this invention to provide an improved twin impeller, jet propulsion unit for watercraft.
It is a further object of this invention to provide an improved, compact twin jet propulsion unit for a watercraft.
Some twin impeller, jet propulsion units have been proposed in which a reversing transmission is provided between the two impellers for effecting counter-rotation while permitting driving from a single prime mover. However, the provision of the transmission within the water flow path obviously either increases the size of the unit and/or decreases the effective pumping area for a given volume.
It is, therefore, a still further object of this invention to provide an improved compact transmission arrangement for a jet propulsion unit wherein the transmission is not disposed in the water flow path.
Another disadvantage of providing the transmission between the two impellers is that the impellers become spaced from each other and this may require or make desirable the use of straightening vanes between the impellers. In addition, the large difference in the distance between the impellers can also necessitate the use of straightening vanes in the discharge. Hence, one of the main advantages of using twin impellers is lost.
It is, therefore, a still further object of this invention to provide an improved twin impeller, jet propulsion unit for watercraft wherein the impellers are disposed immediately adjacent each other and the use of straightening vanes is substantially eliminated.
In many types of watercraft wherein jet propulsion units are employed, the watercraft itself is small and compact in nature. Therefore, there is not adequate room for separate driving arrangements for each of the impellers, nor is there room for large transmissions for driving the impellers in their opposite directions.
It is, therefore, a still further object of this invention to provide an improved, compact and single engine driven counter-rotating jet propulsion unit for watercraft.